Adjustment-indicating variable resistor



y 19, 1964 R. E. CADDOCK ETAL 3,134,036

ADJUSTMENT-INDICATING VARIABLE RESISTOR Filed March 13, 1963 I4 .Ffz-a. 2.

20W Z)E INVENTORS RICHARD E CADDOCK,

y MARLAN E. BOURA/S United States Patent 3,134,086 ADJ USTMENT-INDICATING VARIABLE RESISTOR Richard E. Caddock and Marian E. Bourns, Riverside, Calif., assignors to Bourns, Inc. Filed Mar. 13, 1963, Ser. No. 264,883 12 Claims. (Cl. 338-196) The invention herein disclosed pertains to potentiometers, and more particularly to potentiometers of the socalled multi-turn-rotary type in which a movable contact is caused to brush along a helically arranged resist ance element. In particular, the invention relates to means in combination with the potentiometer for indicating the number of full turns and the fractional part of a turn through which the contact has been moved from a starting position or zero. Still more particularly the invention relates to means of the type noted which indicate to a person manipulating or adjusting the potentiometer, whether the potentiometer contact is at a position completely remote from the zero position (that is, with all of the resistance of the element connected between the contact and the zero terminal) or at a position with none of the resistance of the element so connected.

In a type of potentiometer now marketed, in which a helical plural-turn resistance element is disposed inside a knob-like cylindrical shell that is manually rotated to effect adjustment of the resistance exhibited between the contact and the element terminals, there is provided at the forward end of the shell a turns-indicating structure that is similar in appearance to the face of a time-clock having a dial having a circularly disposed series of numerical indicia, and two indicator-pointers similar to the hands of the clock. Therein as the shell is rotated the longer pointer or hand rotates synchronously with the shell and thus is disposed to cooperate with the dial to furnish precise indications of the fractional parts of incomplete revolutions of the potentiometer contact, and the shorter pointer or hand is geared to the other hand to progress from one numerical indicia to the next each time the longer pointer rotates through one complete revolution (360). The numerical indicia are arranged with a zero (0) at the twelve-oclock position and with digits uniformly spaced around the dial and of number equal to the number (N) of full helical convolutions of the active portion of the resistance element. Such an adjustment-indicating potentiometer is illustrated in US. Patent No. 3,069,646, dated Dec. 18, 1962.

In the cited potentiometer, when the movable contact is adjusted to the low end of the resistance element, so that zero value of resistance is exhibited between the contact terminal and the low-potential end-terminal of the resistance element, the shorter indicator hour hand is at the zero (0) position, indicating that zero full turns of the element are interposed in the circuit between the contact terminal (CT) and the low-potential element terminal (LPT); and under the same condition, the longer or minute hand similarly is positioned directly over the shorter hand. As the potentiometer is then adjusted so as to connect increasing amounts of resistance in circuit between the LPT and the CT, the long hand indicates fractional portions of single turns or convolutions of the element thus connected, and the shorter hand is moved more slowly (similar to the hour hand of a clock) and indicates the number of full turns of resistance in circuit. Continued adjustment of the contact toward the highpotential end of the resistance element will finally result in 100% of the element resistance in circuit between the LPT and the CT; and at that time, both of the hands or indicator limbs will again be in superposition over the O-mark of the dial. Thus there is no diiference in the indication provided for 0% resistance in circuit at one ice extreme of adjustment and that provided for resistance in circuit at the other extreme of adjustment.

It is, in certain instances in employment of such pluralturns variable resistors or potentiometers, very desirable to have provided an easily-discernible difference between the indication for 0% tresistance and that for 100% resistance in the LPT to CT circuit, whereby the technician need not manually manipulate or change the condition of the device to determine in which condition the circuit exists.

Accordingly, it is a prime object of the present invention to provide a variable resistor comprising a pluralturn helical element and a contact movable to contact the element at any selected position between the electrical ends of the element, and means to furnish accurate indications of the position of the contact relative to one end of the element and distinctly difierent indications, respectively for the 0% resistance position and for the 100% resistance position, of the contact.

A second object of the invention is to provide improvements for indicator devices for multi-turn variable re sistors.

A third object of the invention is to provide a clock type dial indicator for indicating differently the positions of the contact of a knob-enclosed helical-element variable resistor in the 0% and 100% resistance conditions, respectively.

Other objects of the invention will hereinafter be stated in the appended claims or made evident in the following description of an exemplary preferred embodiment of the invention as illustrated in the accompanying drawings forming a part of this description.

In the drawings:

FIGURE 1 is a face view of a preferred exemplary variable resistor according to the invention, to a scale chosen for clarity of portrayal only;

FIGURE 2 is a side view of the variable resistor depicted in FIGURE 1;

FIGURE 3 is a fragmentary sectional view of a front portion of the variable resistor depicted in FIGURES 1 and 2, taken as indicated by line 3-3 in FIGURE 1, but grossly enlarged to better illustrate details of construction;

FIGURE 4 is a diagram indicating positional dispositions of indicator devices under opposite extremes of adjustment of a variable resistor according to the invention;

FIGURE 5 is a pictorial representation of gearing depicted in section in FIGURE 3, with portions removed in the interest of clarity of illustration;

FIGURE 6 is an oblique view of a modified form of the invention, with portions broken away; and

FIGURE 7 is a fragmentary view partly in section, showing the relationship of parts depicted in FIGURE 6.

There is chosen, for illustration of the invention, a multi-turn variable resistor of the type disclosed in the aforementioned US. Patent No. 3,069,646; and for that reason and to facilitate full and complete disclosure of details of construction without burdening the present disclosure with details of prior art, the disclosure of that patent is incorporated herein by reference. To further facilitate correlation of disclosures, the same reference numerals or ordinals as those used in the patent will be employed in the present description insofar as the parts are the same; and parts of the presently described structure that are similar to but different from those described in the patent will bear corresponding numerals with a prime added.

Referring to FIGURES 1 and 2, the operating parts of the variable resistor are enclosed and protected by a cylindrical shell 14 that is rotatable about an axis defined by fixed structure including a supporting stud 10. The stud 10 is utilized for mounting and supporting the variable resistor on the front face of a panel or console, and for providing a protected single passage through which element-termination and contact-terminating conductors extend to J-hook terminals 16c, 16 and 30f, respectively. A transparent window or crystal, 32, is sealed in the outer or front end of the shell 14 for protection of the operating parts and to permit visual observation of indicator means that include a plate 20a bearing a series of numerical indicia 20i and first and second pointers 34 and 36. Pointer 34, employed with the indicia 20i to provide indications of rotation of the shell 14 (and of the resistor contact) through angles of less than 360 from a reference point, may hereinafter be termed the long pointer, and similarly pointer 36 may be termed the short pointer. As is evident from the introductory portion of this specification, it is desired to make provisions whereby the short pointer will not point at the zero of the numerical indicia. Rather, it is the object to prevent the short pointer from indicating exactly Zero, that is, to cause it to point to one side of the mark when the resistance in circuit is of zero value, and to point to the opposite side of the zero mark when 100% of the resistance is in circuit. With the short pointer thus arranged, the technician or operator of the variable resistor can readily by visual inspection alone, determine whether the resistance in circuit is 0% or 100%.

The indicator means and operating structure therefor are supported at the front end of the variable resistor by a pin 20s (FIGURE 3) that is fixedly secured in a bore 181? provided in a plug 18 comprised in stationary structure of the resistor. Positioned by pin 20s and held against the front face of plug 18 is a plate 2015 that is companion to plate 20a. The two plates are held in spaced-apart parallel relationship and plate 20a is supported by plate 20b, by suitable spacing means such as welded deformations 201", of which three or more may be provided, equally spaced about the axis of pin 20s. Rotatably supported by the previously mentioned locating pin or stud 20s is a fractional-turn indicator pointer 34 the outer end of which engages in a slot 14s (FIGURE 3, upper left) formed in the inner periphery of shell 14. The outer end of the pointer 34 may be made resilient, to accommodate some eccentricity of shell 14 with respect to the axis furnished by pin 20s, and the pointer is constructed and arranged to be driven (rotated) incident to rotation of the shell 14. The inner end of pointer 34 is afiixed to the front end of a sleeve hub 38 that is journaled on pin 20s; and the rear end of hub 38 has afiixed thereon a pinion 39. Pinion 39 is meshed with an idler gear 40 that is rotatably mounted on a gudgeon 42 affixed to plate 20b; and gear 40 has affixed thereto for rotation therewith a pinion 40p. Pinion 40p is meshed with a spur gear 44 which has an elongate hub disposed as indicated for rotation upon hub 38. Gears 39, 40-401), and 44 are disposed between plates 20a, 20b, and the elongate hub of gear 44 extends forwardly through a central aperture in plate 20a and has afiixed thereto for rotation therewith a full-turn indicator pointer 36'. The ratios of the gears and the arrangement of indicia 201' will vary according to the number of full turns in the electrically effective portion of the helical resistance element. In the illustrative exemplary embodiment of a potentiometric device according to the invention, there are ten turns of effective extent of the element, and accordingly the numeric indicia provide for ten divisions in the dial (plate 20a) of the indicating means; and the gearing ratios are such that one complete revolution of driving pointer 34 causes rotation of pointer 36' through slightly less than one-tenth of one revolution.

The ratio of the gearing formed by the previously described gearing, and the relative positioning of the pointers is such that when the short pointer is. precisely over the 50% marker (numeral in the exemplary ten-turn potentiometer), the long pointer is precisely over the 0 mark; but the ratio is such that as the potentiometer is adjusted from midposition toward either extreme, the long pointer indicates precisely the fractional portions of each full turn of adjustment, but the short indicator lags very slightly behind. Thus as the resistor is adjusted from the 50% position toward the 0% position, the short pointer points toward numerals 4, 3, 2, and l with a satisfactory degree of accuracy (since only a general indication is required of that pointer), and when the adjustment proceeds through the last turn of adjustment, the short pointer approaches the zero position but has lagged sufficiently that it is only near that position when the long pointer reaches Zero. The relative positions of the pointers when the 0% resistance adjustment is reached is indicated in full lines in FIGURE 4. While different ratios of gearing may be used, depending upon the diameter of the dial (plate 20a), for a diameter of inch, a ratio of 10 full turns (3600") of pointer 34 to 0.9677 turn of pointer 36 has been found to be satisfactory. In the illustrated gearing that ratio may be attained with pinion 39 having 12 teeth, gear 40 with 31 teeth, pinion 40p With 8 teeth, and gear 44 with 32 teeth, as indicated in FIGURE 5.

With the gearing in place in the instrument and the shell rotated to a position at which the contact is exactly five turns from either electrical end, pointer 36' is applied on the hub of gear 44 and pointing precisely to the midpoint of the indicia (numeral 5 in the example). Thereafter, pointer 34 is applied on hub 33 and pointing at the zero indicia. Thereafter, the crystal 32 is cemented in place. Thus, as the shell 14 is rotated to increase the resistance between the contact and the low-potential terminal, pointer 34 will as before accurately indicate, with the aid of the indicia including the numerals and inter-numeral points on the dial, the fractional portion of 360 through which the adjustment has proceeded from the zero point of each turn, and pointer 34 will indicate very satisfactorily with which turn or convolution of the element the contact is in engagement, until finally and as the increasing adjustment rotation of shell 14 is stopped as pointer 34 is again over the 0 on the dial, pointer 36' will be positioned so as to point slightly to the left of pointer 34, as indicated by the dot-dash line in FIGURE 4.

The preceding description, in conjunction with the illustrations in the drawings, make it evident that the indicating means is such as to provide the technician unambiguous and unmistakable indications of both of the 0% and adjustments of the variable resistor. The prior art devices, as exemplified by the variable resistor disclosed in the aforementioned patent, fail to provide such indications. For example, in the patented device, due to the N to 1 ratio (10 to 1 in the exemplary ten-turn instrument), the short pointer was disposed directly behind the long pointer 34 in both the 0% and the 100% adjustment conditions. As will be evident to those skilled in the art, the deviation of pointer 36' from the zero position is of considerable value to the person contemplating adjustment of the resistor from either extreme, while the deviation of that pointer from precise indication of the numerals on the dial is of no consequence.

In FIGURES 6 and 7 a modified form of the invention is illustrated, in which form a small stop device in the form of a fixed pin prevents the short pointer from being brought to the exact zero position on the dial. In the modified form of the invention the gearing may assume a N to 1 ratio (10 to l in the illustrated ten-turn resistor), and slippage between pointer 36 and the hub of gear 44 occurs during the terminal stages of adjustment of the resistor in the decreasing and increasing directions. To that end a rigid pin 20p (FIGURE 7) is set in a suitable bore formed in plate 20a at a point in line between the 0 indicia and the axis of pin 20s. The radial position of pin 20p is dependent upon the diameter of the dial, the width of pointer 36', and the diameter of the pin, and may be selected accordingly but such that pointer 36' can be driven close to the Zero position in both directions without reaching that position. The length of the protruding or exposed portion of pin 20p is such as to effectively stop pointer 36 but insuflicient to interfere with pointer 34. The relationship is illustrated in FIGURE 7. As pointer 36 is driven toward a position in which it would point to the numeral 0 on the dial, it is driven into engagement with pin 20p and is arrested. Continued driving torque is exerted uponthe hub of gear 44 however, as pointer 34 is driven to the 0 position, and during that interval the hub slips relative to pointer 36'. Such slippage is permitted by the friction fit of the pointer on the hub. Due to pointer 34 stopping at 0 incident to action of mechanical stops means in the potentiometer, the displacement caused by the slippage of the pointer on the hub of gear 44 is very small and is of no significance or consequence in reading'the indications of the resistor. As is evident, a similar result may be attained by making pointer 36' resilient so that the outer end of the pointer yields when driven against stop pin 20p. Thus it is seen that pin 20p serves, like the gearing depicted in section in FIGURE 3, as a means for preventing the full-turns indicator means (pointer 36) from reaching a single terminal position from both directions of approach.

The preceding description of the invention, while indicating principal and subsidiary modes and apparatuses for accomplishing the desired results and indicating complete attainment of the objects, is nevertheless directed only to a presently preferred physical embodiment of the invention. In the light of the present disclosure it is apparent that changes and modifications within the spirit of the invention will occur to those skilled in the art, and accordingly we do not desire that the invention be restricted to the details of the described embodiment.

We claim: 1. An adjustment-indicating variable resistor compris- 1ng:

first means, including a resistance element device having N helically-arranged convolutions of electrically active resistance element between the low-potential and high-potential electrical ends thereof, a contact, and rotary means arranged for rotation through N revolutions to cause relative rotation between said element and said contact to cause the contact to wipe from one electrical end of said element to the other thereof to vary the resistance exhibited between said contact and an electrical end of said element;

second means, including indicia-bearing means bearing a circular array of indicia including N numerical indicia including a zero, said second means including power-transmission means and indicator means comprising first and second indicator devices arranged for visual observation and disposed closely adjacent to said indicia-bearing means and arranged with said indicia-bearing means for relative rotation therebetween whereby to provide indications of the extent of rotation of said rotary means, relative rotation between said first indicator device and said indiciabearing means being at the rate of one revolution for each convolution of resistance element wiped by said contact, and relative rotation between said second indicator device and said, indicia-bearing means being restricted by means included in said second means to slightly less than 360 for N convolutions of resistance element wiped by said contact; and

third means fixing said second means relative to said first means so that when said contact is at the lowpotential electrical end of said element said first indicator device is directed to said zero and said second indicator is directed slightly ahead of said zero,

whereby to provide an unambiguous indication that said variable resistor is adjusted to the low-potential end of its range of adjustment.

2. An adjustment-indicating variable resistor according to claim 1, in which said rotary means comprises a generally cylindrical shell in which said resistance element is housed.

3. An adjustment-indicating variable resistor according to claim 2, in which said first indicator device is a pointer directly driven by said cylindrical shell, and arranged for rotation thereby through N revolutions incident to traverse of said contact over the said N convolutions of said element, and in which said indicia-bearing means comprises a dial on which said indicia are visibly arrayed to be passed by said pointer during movement of the pointer.

4. An adjustment-indicating variable resistor according to claim 3, in which said power-transmission means includes gearing interposed between said pointer and said second indicator device to drive the latter, and in which said second indicator device is a second pointer and in which the ratio of said gearing is l/N and in which said second means includes stop means effective to prevent said second pointer from fully completing a traverse of all of said N numerical indicia.

5. An adjustment-indicating variable resistor according to claim 3, in which said second indicator device is a second pointer rotatably mounted for rotation over said dial, and in which said power-transmission means includes gearing interposed between said first and second pointers to drive the latter incident to movement of the first pointer, and in which said gearing has a ratio of less than l/N, whereby said second pointer is rotated less than one complete revolution incident to rotation of said first pointer through N revolutions.

6. An adjustment-indicating potentiometer, comprising:

first means, comprising a helical resistance element having a low-potential end and a high-potential end and comprising N full convolutions between said ends, a contact arranged to wipe on said element, structural means for inducing relative rotation between said contact and said element whereby said contact wipes along said element between said ends and makes a full traverse therebetween incident to N complete revolutions of said relative rotation, said structural means comprising a rotary means rotatable about the axis of said helical resistance element, and said first means comprising means restricting said relative rotation to N complete revolutions in either direction from a respective extreme limit of relative rotation;

second means, comprising dial indicator means including a dial disposed generally perpendicularly to the axis of said rotary means, and including first and second rotary pointers arranged for rotation about an axis perpendicular to said dial, said dial comprising indicia including a zero and said indicator means comprising power-tranmission means interposed between said first and second rotary pointers and effective to provide a rotation ratio therebetween selected from among the ratios N /1 and N X wherein X is of numerical value less than one; and

third means providing a' power-transmitting connection between said first means and said second means;

.Whereby incident to rotation of said rotary means said first pointer traverses one full revolution around said dial incident to traverse of each full convolution of said element by said contact and whereby incident to traverse of said contact from end to end of said element through said N convolutions of the latter said second pointer is rotated through slightly less than 360 around said dial.

7. An adjustment-indicating variable resistor comprismg:

first means, including a multi-turn rotary variable resistor having a stationary support means and a rotatable generally cylindric device deriving support from the support means and constructed and arranged to be manually rotated through N complete revolutions and thereby effect adjustment of the variable resistor '7 through the range of adjustment thereof from either limit of adjustment to the other; and

second means, including indicator means supported by said first means and mechanically connected thereto for actuation, said indicator means comprising dial means providing a circular array of visible indicia including a reference indicia, and said indicator means including first and second rotatable pointer devices and gearing therebetween with means for supporting said pointers for rotation about said axis in close proximity to said dial means, and said second means including a mechanical connection between said rotatable generally cylindric device and a selected component of the group of components comprising said gearing and said pointers whereby incident to rotation of said cylindric device said first pointer is rotated in unison therewith around said axis past the indicia on said dial and provides an accurate indication of extent of rotation of said first pointer device from said reference indicia, said gearing having a ratio such that rotation of said first pointer device through N complete revolutions is: concurrent with rotation of said second pointer device through 360X wherein X is selected from values between 1 and 8. An adjustment-indicating variable resistor, comprising:

first means, including a multi-turn variable resistor comprising support means adapted to be secured to a panel for support and comprising a knob-like device rotatably supported on the support means and effective by rotation through N revolutions to adjust the resistor from either limit of adjustment to the other;

second means, comprising stationary dial means bear ing a circular array of numerical indicia thereon and including a reference indicia, supported by said support means;

third means, including first and second rotatable pointers and means supporting the pointers for rotational movement past said indicia; and fourth means, comprising power-transmitting speedchanging means interconnecting said first and second rotatable pointers and providing between the said first and second pointers a speed reduction of 1/ N iX wherein N is the number of complete revolutions of said knob-like device required to adjust the variable reistor from the lower limit of adjustment thereof to the upper limit of adjustment thereof and wherein X is a number whose value is a small fraction of the value of N, said first means and said fourth means providing means for rotating said first pointer substantially in unison with said knob-like means, and said first pointer arranged for registry with said reference indicia on said dial means incident to adjustment of the variable resistor to the lower limit thereof and said power-transmitting speed changing means being arranged to concurrently move said second pointer to a position X/2 revolution removed in one direction from said reference indicia and being arranged to move, incident to adjustment of the variable resistor to the upper limit thereof, to a position X 2 revolution removed in the opposite direction from said reference indicia,

whereby when said Variable resistor is adjusted to either limit thereof said second pointer is positioned X/2 revolution from said indicia and provides an unambiguous indication of the particular limit to which the resistor is adjusted.

9. A variable resistor according to claim 8, in which said knob-like device is a generally cylindrical shell and in which the outer end of said first pointer is connected to said shell to be rotated thereby in both directions.

10. A variable resistor comprising a knob-like shell and means supporting the shell for rotation about an axis, op-

253 erating means within said shell supported by said supporting means and operated by said shell incident to rotation of the shell through N complete revolutions to effect adjustment of resistance exhibited by the resistor between upper and lower limits, adjustment-indicating means comprising a dial and first and second pointers rotatable around the face of said dial, said dial bearing a generally circular array of indicia, driving means effective to drive said first pointer in unison with rotational movement of said shell and said first pointer registering with a certain one of said indicia at either limit of adjustment of the resistor, said adjustment-indicating means comprising means for rotatably supporting said first and second pointers and also comprising gearing means interconnecting said pointers and having the driving ratio 1/N+X wherein X is a small fraction of N, so that incident to rotation of said first pointer through said N complete revolutions said second pointer is rotated through slightly less than one complete turn, whereby the respective different positions of said second pointer relative to said indicia at the upper and lower limits of adjustment of said resistor furnish respective easily distinguished indications of the state of adjustment of the variable resistor at said limits.

11. A variable resistor of the rotary multi-turn type adjusted through the entire range thereof between upper and lower adjustment limits incident to rotation of adjustment means through N turns in which N is a whole number in excess of one, said resistor comprising adjustment means including a knob-like device, an indicator dial bearing indicia, first and second rotatable indicators movable relative to said dial to indicate, respectively, fractional portions of each complete 360 rotation of said adjustment means in one direction, and number of complete 360 rotations of said adjustment means in said one direction, gearing means interconnecting said first and second rotatable indicators, said gearing means effective to drive said second indicator from said first indicator with an effective speed ratio equal to l/N+X wherein X is a small fraction of N, and said variable resistor comprising means for rotating said first indicator in unison with said adjustment means.

'12. An adjustment-indicating variable resistor, comprising:

first means, including a multi-turn variable resistor comprising support means adapted to be secured to a panel for support and comprising a knoblike device rotatably supported on the support means and effective by rotation through N revolutions to adjust the resistor from either limit of adjustment to the other; second means, comprising stationary dial means bearing a circular array of numerical indicia thereon and including a reference indicia, supported by said support means;

third means, including first and second rotatable pointers and means supporting the pointers for rotational movement past said indicia; and

fourth means, comprising poweratransmitting speedchanging means interconnecting said first and second rotatable pointers and providing between the said first and second pointers a speed reduction of l/NiX wherein N is the number of complete revolutions of said knob-like device required to adjust the variable resistor from the lower limit of adjustment thereof to the upper limit of adjustment thereof and wherein X is a number whose value is a small fraction of the value of N, said first means and said fourth means providing means for rotating said first pointer substantially in unison with said knob-like means, and said first pointer arranged for registry with said reference indicia on said dial means incident to adjustment of the variable resistor to the lower limit thereof and said power-transmitting speed changing means being arranged to concurrently move said second pointer to a position X 2 revolution removed in one direction from said reference indicia and being arranged to move, incident to adjustment of the variable resistor to the upper limit thereof, to a position X/ 2 revolution removed in the opposite 5 direction from said reference indicia,

whereby when said variable resistor is adjusted to either limit thereof said second pointer is positioned X 2 revolution from said reference indicia and provides an unambiguous indication of the particular 10 limit to which the resistor is adjusted.

References Cited in the file of this patent UNITED STATES PATENTS Price Dec. 13, 1904 Harris Apr. 10, 1923 Bourns Dec. 23, 1958 J-atfe May 26, 1959 Hardison et al. Dec. 18, 19 62 FOREIGN PATENTS Great Britain Apr. 23, 1925 

1. AN ADJUSTMENT-INDICATING VARIABLE RESISTOR COMPRISING: FIRST MEANS, INCLUDING A RESISTANCE ELEMENT DEVICE HAVING N HELICALLY-ARRANGED CONVOLUTIONS OF ELECTRICALLY ACTIVE RESISTANCE ELEMENT BETWEEN THE LOW-POTENTIAL AND HIGH-POTENTIAL ELECTRICAL ENDS THEREOF, A CONTACT, AND ROTARY MEANS ARRANGED FOR ROTATION THROUGH N REVOLUTIONS TO CAUSE RELATIVE ROTATION BETWEEN SAID ELEMENT AND SAID CONTACT TO CAUSE THE CONTACT TO WIPE FROM ONE ELECTRICAL END OF SAID ELEMENT TO THE OTHER THEREOF TO VARY THE RESISTANCE EXHIBITED BETWEEN SAID CONTACT AND AN ELECTRICAL END OF SAID ELEMENT; SECOND MEANS, INCLUDING INDICIA-BEARING MEANS BEARING A CIRCULAR ARRAY OF INDICIA INCLUDING N NUMERICAL INDICIA INCLUDING A ZERO, SAID SECOND MEANS INCLUDING POWER-TRANSMISSION MEANS AND INDICATOR MEANS COMPRISING FIRST AND SECOND INDICATOR DEVICES ARRANGED FOR VISUAL OBSERVATION AND DISPOSED CLOSELY ADJACENT TO SAID INDICIA-BEARING MEANS AND ARRANGED WITH SAID INDICIA-BEARING MEANS FOR RELATIVE ROTATION THEREBETWEEN WHEREBY TO PROVIDE INDICATIONS OF THE EXTENT OF ROTATION OF SAID ROTARY MEANS, RELATIVE ROTATION BETWEEN SAID FIRST INDICATOR DEVICE AND SAID INDICIABEARING MEANS BEING AT THE RATE OF ONE REVOLUTION FOR EACH CONVOLUTION OF RESISTANCE ELEMENT WIPED BY SAID CONTACT, AND RELATIVE ROTATION BETWEEN SAID SECOND INDICATOR DEVICE AND SAID INDICIA-BEARING MEANS BEING RESTRICTED BY MEANS INCLUDED IN SAID SECOND MEANS TO SLIGHTLY LESS THAN 360* FOR N CONVOLUTIONS OF RESISTANCE ELEMENT WIPED BY SAID CONTACT; AND THIRD MEANS FIXING SAID SECOND MEANS RELATIVE TO SAID FIRST MEANS SO THAT WHEN SAID CONTACT IS AT THE LOWPOTENTIAL ELECTRICAL END OF SAID ELEMENT SAID FIRST INDICATOR DEVICE IS DIRECTED TO SAID ZERO AND SAID SECOND INDICATOR IS DIRECTED SLIGHTLY AHEAD OF SAID ZERO, WHEREBY TO PROVIDE AN UNAMBIGUOUS INDICATION THAT SAID VARIABLE RESISTOR IS ADJUSTED TO THE LOW-POTENTIAL END OF ITS RANGE OF ADJUSTMENT. 